FIG. 9 is a perspective view illustrating a prior art photodiode integrated short resonator length semiconductor laser device conceived from the structure described in Electronics Letters, volume 18, pages 189-190 (1982). In FIG. 9, reference numeral 1 designates a semiconductor substrate. A semiconductor laser 2 is disposed on the semiconductor substrate 1. The semiconductor laser 2 has a front side laser beam emitting facet 3 which includes a front side laser beam emitting point 4. The semiconductor laser 2 has also a rear side laser beam emitting facet 5 which includes a rear side laser beam emitting point 6. Reference numeral 7 designates an upper surface electrode of the semiconductor laser 2 and numeral 8 designates a bonding pad of the upper surface electrode 7. A photodiode 9 that monitors the laser beam emitted from the rear side laser beam emitting point 6 is disposed on the semiconductor substrate 1. The photodiode 9 has a light responsive surface 10 that includes a light responsive point 10a. Reference numeral 11 designates an upper surface electrode of the photodiode 9 and numeral 12 designates a lower surface electrode common to the semiconductor laser 2 and the photodiode 9.
In this prior art example, the length of the semiconductor laser 2, i.e., the resonator length L is short, for example, 20 .mu.m, and it is difficult to bond the upper surface electrode 7 onto the semiconductor laser 2. Therefore, a bonding pad 8 is connected with the upper surface electrode 7 of the semiconductor laser 2 that is disposed beside the semiconductor laser 2. The photodiode 9 is disposed at a position where the light responsive surface 10 is opposed to the rear side laser beam emitting facet 5 as shown in FIG. 9.
The front side laser beam emitting facet 3 and the rear side laser beam emitting facet 5, a pair of reflecting mirror surfaces of the semiconductor laser 2, are formed perpendicular to the surface of the substrate 1 by a dry etching process such as RIE (reactive ion etching) or RIBE (reactive ion beam etching).
The laser beam emitted from the laser beam emitting point 6 of the rear side laser beam emitting facet 5 is incident on the photodiode light responsive surface 10 to generate a monitor signal at the photodiode 9. In addition, a portion of the laser beam incident on the light responsive surface 10 of the photodiode 9 is reflected by the light responsive surface 10.
When the light responsive surface 10 of the photodiode 9 is perpendicular to the surface of the substrate 1, a portion of the laser beam reflected by the light responsive surface 10 returns to the rear side laser beam emitting point 6, and induces return light noise in the semiconductor laser 2. In the Electronic Letters publication, in order to avoid this problem, the light responsive surface 10 of the photodiode 9 is not perpendicular to the substrate 1 but is inclined with respect to the substrate 1.
FIG. 10 is a diagram illustrating a semiconductor laser array device in which a semiconductor laser array comprising a plurality of semiconductor lasers arranged in an array and a photodiode array comprising a plurality of photodiodes arranged in an array, each of which monitors the laser beam emitted from one facet of one of the semiconductor lasers, are disposed on the same semiconductor substrate parallel to each other.
In FIG. 10, the same reference numerals as in FIG. 9 designate the same or corresponding parts. Reference numeral 20 designates a semiconductor laser array disposed on the substrate 1, numerals 21a and 21b designate semiconductor lasers forming the semiconductor laser array 20, numeral 22 designates a front side laser beam emitting facet of the semiconductor laser array, numerals 23a and 23b designate front side laser beam emitting points of the semiconductor lasers 21a and 21b, respectively, numeral 24 designates a rear side laser beam emitting facet of the semiconductor laser array, and numerals 25a and 25b designate rear side laser beam emitting points of the semiconductor lasers 21a and 21b, respectively. Reference numeral 30 designates a photodiode array disposed on the substrate 1. Reference numerals 31a and 31b designate photodiodes, each of which corresponds to one of the semiconductor lasers 21a and 21b and monitors the laser beam emitted from the corresponding rear side laser beam emitting points 25a and 25b of the semiconductor lasers 21a and 21b, and forming the photodiode array 30. Reference numerals 32a and 32b designate light responsive surfaces of respective photodiodes 31a and 31b, respectively.
A description is given of the operation of a semiconductor laser and photodiode pair. The laser beam emitted from the rear side laser beam emitting point 25a of the semiconductor laser 21a is incident on the light responsive surface 32a of the photodiode 31a which corresponds to the semiconductor laser 21a and generates a monitor signal in the photodiode 31a.
FIG. 11 shows a manner of coupling between a semiconductor laser device and an optical fiber according to the prior art. In FIG. 11, the same reference numerals as in FIG. 9 designate the same or corresponding parts. Reference numerals 41 and 42 designate lenses and numeral 43 designates an optical fiber. The lenses 41 and 42, and the optical fiber 43 are accurately positioned so that a reduction in the coupling efficiency when the laser beam emitted from the front side laser beam emitting point 4 of the semiconductor laser 2 is introduced into the optical fiber 43 is prevented.
The laser beam emitted from the rear side laser beam emitting point 4 of the semiconductor laser 2 is first incident on the lens 41, whereby it becomes a parallel light beam. Then, the parallel light beam is incident on the lens 42 and collimated and the collimated laser beam is incident on the optical fiber 43.
The prior art photodiode integrated semiconductor laser device constructed as described above has the following problems. Initially, in the semiconductor laser device shown in FIG. 9, the light responsive surface 10 of the photodiode 9 is inclined with respect to the surface of the substrate 1, not perpendicular to the surface of the substrate 1. Therefore, the light responsive surface 10 of the photodiode 9 cannot be produced at the same time as the front side laser beam emitting facet 3 and the rear side laser beam emitting facet 5, that are perpendicular to the surface of the substrate 1. Therefore, it was necessary to produce the facets in a separate process.
As a method for forming a surface having an inclination with respect to the surface of the substrate 1 as the light responsive surface 10 of the photodiode 9, a surface that is perpendicular to the surface of the substrate 1 may be formed using the same method used in forming the front side laser beam emitting facet 3 and the rear side laser beam emitting facet 5. Thereafter, by FIB (focused ion beam) or wet etching, only the light responsive surface 10 of the photodiode 9 is made to incline. However, these methods require separate processes.
In the semiconductor laser array device shown in FIG. 10, the laser beam emitted from the rear side laser beam emitting point 25a of the semiconductor laser 21a is to be received only by the light responsive portion 32a of the corresponding photodiode 31a and monitored, and, similarly, the laser beam emitted from the rear side laser beam emitting point 25b of the semiconductor laser 21b is to be received only by the corresponding light responsive portion 32b of the photodiode 31b and monitored. However, crosstalk arises in which a portion of the laser beam emitted from the rear side laser beam emitting point 25b of the semiconductor laser 21b is received by the light responsive surface 32a of the photodiode 31a and monitored.
In order to suppress crosstalk, the separation between respective photodiodes arranged in an array may be increased. However, the degree of integration in the photodiode array is then reduced.
In the semiconductor laser device shown in FIG. 11, in order to couple the laser beam emitted from the front side laser beam emitting point 4 of the semiconductor laser 2 efficiently to the optical fiber 43, it is required to position the lens 41, the lens 42, and the optical fiber 43 accurately, which is troublesome work.